This invention relates to the field of inspection and particularly to the field of ultrasonic inspection of materials.
Conventional ultrasonic inspection techniques utilize a narrow beam of longitudinal or transverse type ultrasonic waves which is injected into the part being inspected by contacting the transducer with the part or by contacting the transducer with a transmitting medium, such as water, which also contacts the part. Such contact transducers generally employ piezoelectric crystals.
More recently, electromagnetic acoustic transducers (EMATs) have been developed which are capable of injecting ultrasonic waves into the part without any physical contact with the part. Because of the non-contact feature of such transducers, they are particularly useful in many nondestructive testing applications. One such electromagnetic transducer utilizes a periodic meander coil placed in a magnetic field. When an RF signal is applied to the meander coil, an ultrasonic wave is created in the test part, as described in U.S. Pat. No. 3,850,028.
A later developed type of electromagnetic acoustic transducer does not utilize a periodic meander coil. Rather, this type of transducer includes a row or stack of individual, alternately oriented permanent magnets which create a static, periodic magnetic field. One side of a coil is placed in the periodic magnetic field so that a sheet of current moves transverse to the magnetic field when a pulse of current flows through the coil. This latter type of transducer can be used to create horizontal shear (SH) waves and Lamb waves in an electrically conductive part, as described in U.S. Pat. No. 4,127,035.
The various EMATs described above are built with a particular periodicity D (determined by the spacing of a meander coil or of a row of magnets) which defines the wavelength, .lambda., of the ultrasonic wave which the EMAT is designed to generate. The frequency, f, used to generate the ultrasonic wave is then determined from the dispersion relationship between the wavelength, .lambda., and the velocity, v, of the wave in the particular material of the part being tested. The frequency of the fundamental horizontals hear wave mode, n=0, for example is given by the relationship v=.lambda.f. The relationship is more complex for higher order modes and is based on dispersion curves characteristic of the mode type.
Unfortunately, however, electromagnetic transducers exhibit a low efficiency in operation as compared to piezoelectric transducers, which makes it difficult to use an EMAT to locate small defects. Consequently, methods and apparatus for increasing the operating efficiencies of electromagnetic transducers and other transducers which operate with reduced efficiency are needed in the art.